A diesel vehicle is widely used due to its excellent gas mileage and small CO2 discharge amount compared to a gasoline vehicle. However, nitrogen oxides (NOx) and particulate matter (PM) which are harmful to a human body and an environment are included in the exhaust gas of the diesel vehicle. Main regulation for diesel vehicle exhaustion is on the nitrogen oxides and particulate matter. Particularly, nitrogen oxide is a main cause of smog and acid rain, and thus, the regulation is being tightened. Therefore, technologies for removing nitrogen oxides from exhaust gas of automobiles, which are main causing materials of smog and acid rain and harmful to a human body and an environment, have been developed.
Conventionally, a selective catalytic reduction (SCR) method has been used to remove nitrogen oxides discharged from diesel engines, wherein the nitrogen oxides are converted to nitrogen and treated with a catalyst using a reducing agent. The selective catalytic reduction method selectively reduces the nitrogen oxides and converts to nitrogen using urea, hydrocarbon, and the like as the reducing agent and using a zeolite or alumina catalyst, and the like. Nitrogen oxide removal performance of Urea/SCR technology is known to be excellent. However, a zeolite catalyst used in the Urea/SCR technology has a low hydrothermal stability, a catalytic activity is easily lowered by SO2 and hydrocarbons included in exhaust gas, and a storage tank is required for periodical injection of urea.
An HC/SCR technology, which was developed for solving various problems of the Urea/SCR technology, removes nitrogen oxides by directly using hydrocarbons or fuel discharged from exhaust gas as a reducing agent, and has a high catalytic reactivity and selectivity. However, the HC/SCR technology generates ammonia (NH3) which is harmful to human body and cannot be removed with an Ag/Al2O3 catalyst that was widely used before.
It is known that diesel engine exhaust gas temperature of a diesel vehicle is about 150 to 250° C. for a light-duty vehicle, and about 200 to 350° C. for a heavy-duty vehicle (R. G. Gonzales, “Diesel Exhaust Emission System Temperature Test”, T&D Report 0851-1816P, SDTDC, U.S. Department of Agriculture, December (2008)). Thus, a technology for a post-treatment of exhaust gas capable of removing discharged nitrogen oxides at a temperature of 350° C. or less is required. Further, the Ag/Al2O3 catalyst widely used in HC/SCR exhibits an excellent nitrogen oxide removal efficiency at a temperature of 300° C. or more, but exhibits a low activity at a temperature of 300° C. or less.
Accordingly, there is a demand for development of a catalyst having an excellent removal efficiency of ammonia (NH3) generated during a reduction reaction and improved nitrogen oxide removal performance at a low temperature that is a practical temperature of exhaust gas.